DESCRIPTION: Intercellular junctions play an important role in cell proliferation, intercellular barrier formation, differentiation and tissue morphogenesis. In addition they maintain structural integrity of the tissues, and also serve as centers for signal transduction. We have characterized Drosophila Neurexin IV (NRX IV) and showed that this protein is required for the formation of glial-glial septate junctions. These junctions are critical for the maintenance of the blood-brain barrier and for insulation of axons against the hemolymph. Loss of function of NRX IV resulted in loss of septate junctions between glial cells and hence, breakdown of the blood-brain barrier. In vertebrates, septate junctions are present only in myelinated nerve fibers which are organized into anatomically and physiologically distinct longitudinal domains: the internode, the juxtaparanodes, the paranodes and the node. The formation and organization of these structures results from a sequence of molecular events including migration, relocalization and clustering of various proteins into specific domains. The septate-like junctions are present at the paranodes and provide a site of adhesion between the axons and glial cells. They also fence the distribution of voltage gated Na+ and delayed rectifier K+ channels which are highly concentrated in the nodal and juxtaparanodal regions, respectively. Complex molecular interactions between axons and myelinating glial cells are essential for the rapid conduction of nerve impulses via saltatory conduction. We have identified the mouse homolog of Drosophila NRX IV, MNCP (for MNRX IV/Caspr/Paranodin) and show that the subcellular distribution of MNCP is developmentally regulated, and that the MNCP localization to paranodes coincides with the formation of paranodal junctions. MNCP null mice display severe neurologic defects, and die postnatally within 3-4 weeks. Loss of function of MNCP results in loss of transverse bands or septate junctions at the paranodes and causes abnormal nodal morphology. In this grant we propose to use a combination of genetic, molecular and biochemical methods to address the following questions: 1) What molecular events are involved in the formation of paranodal septate junctions and what role MNCP plays in the formation of these junctions? 2) What genes are required for the formation and function of septate junctions in Drosophila? And 3) What are the extracellular ligands/receptors 01 MNCP on the glial surface and what are the intracellular partners that associate with MNCP at the paranodal junctions? These studies should help understand the function of this highly conserved protein family, and provide new insights into the mechanisms that govern the biogenesis of septate junctions from Drosophila to human.